The present invention relates to hydraulic devices such as pumps and motors, and more particularly to such devices in which the fluid displacement mechanism is of the roller gerotor type. Hydraulic devices including displacement mechanisms of the roller gerotor type are sold commercially by the assignee of the present invention under the trademark Geroler.RTM., which trademark is owned by the assignee of the present invention.
Although the present invention may be utilized with any type of hydraulic device having a fluid displacement mechanism of the roller gerotor type, it is especially suited for use with gerotors of the internally-generated rotor (IGR) type, and will be described in connection therewith.
A fluid displacement mechanism of the IGR type is illustrated and described in U.S. Pat. No. 3,623,829, incorporated herein by reference. In an IGR device, there is an inner gear (or inner rotor) defining a plurality N of cylindrical openings, each of which has a cylindrical roller disposed therein. The cylindrical rollers serve as the external teeth of the inner gear. The inner gear is eccentrically disposed within a conjugate, internally-toothed outer gear (or outer rotor) having a plurality N+1 of internal teeth.
An IGR device is especially suited for use in a pump, in which case both the inner gear and the outer gear rotate about their respective axes of rotation. When an IGR device is utilized in a pump, there is no relative orbital rotation between the axes of the gears, as is normally the case in an orbiting gerotor of the type used in a low speed, high torque motor. The primary advantage of an IGR device, when used in a pump, is that centrifugal force on the rollers (the external teeth of the inner gear) causes the roller to seal against the conjugate surface (internal teeth) of the outer gear, thus providing for improved volumetric efficiency.
Despite the advantages noted above, IGR type pumping devices have not been especially successful, commercially. There are two basic design approaches available with IGR pumps. In one design approach, which is referred to as a "fixed clearance" design, the housing members immediately axially adjacent the end surfaces of the gerotor are maintained at a fixed axial separation, thus making it nearly certain that there will be a slight clearance between the end surfaces of the gerotor and the adjacent housing surfaces. Such a clearance inherently limits the performance of the pump. If relatively high volumetric efficiency is desired, the rated pressure of the pump must be relatively lower. Conversely, if it desired to have a relatively higher rated pressure for the pump, the volumetric efficiency will be lower.
The other design approach is to have axially movable balancing members adjacent the axial end surfaces of the gerotor, with the balancing members biased into sealing engagement with the end surfaces of the gerotor, for example, by means of fluid pressure. Typically, in such a design, the balancing is accomplished using the output pressure of the pump. The use of this design approach substantially eliminates the clearances along the axial end faces of the gerotor, thus making it possible to operate the pump at a relatively high rated pressure, while still maintaining relatively high volumetric efficiency.
However, in spite of the theoretical advantages of an IGR pump with pressure biased sealing members, there has apparently not been a commercially successful pump of this design. In connection with the development of the present invention, several possible reasons for such lack of commercial success appeared. The pressure biasing or clamping of the members adjacent the IGR type gerotor results in high speed relative rubbing movement between the gerotor (which is rotating) and the adjacent sealing members (which are stationary). It has been observed that such high speed relative motion results in galling between the ends faces of the outer gear and the adjacent surface of the sealing member. As is well known to those skilled in the art, galling typically occurs when there is a breakdown of, or a total loss of, the fluid film between two relatively rotating, engaged metal surfaces. As is also well known to those skilled in the art, galling between two adjacent, engaged metal surfaces typically leads to destruction or inoperability of the device within a fairly short time.
It was also observed in connection with the development of the present invention that the end surfaces of the rollers would gouge the adjacent surface of the sealing member, sometimes in addition to causing galling, either of which, individually, would also lead to destruction or inoperability of the device in a fairly short time. It has been hypothesized that one cause of the gouging of the adjacent surface was lack of perfect perpendicularity between the end surface of the roller and the axes of the roller, such that the end surface of the roller is not perfectly parallel to the adjacent sealing surface, but instead, a portion of the end surface of the roller gouges or digs into the adjacent sealing surface.
It has been known by those skilled in the art to place a break or chamfer on the corners of the rolls, and even to crown the end surfaces of the rolls, in an attempt to have pressurized fluid acting on the axially opposite ends of the rolls. However, any such attempt at balancing the rolls which involves communicating pressurized fluid from the adjacent volume chambers is, in effect, a leak path which results in a loss of volumetric efficiency.